linux-hardening - Re: [PATCH 35/82] ACPI: custom_method: Refactor intentional wrap-around test

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Date: Wed, 24 Jan 2024 20:52:48 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: linux-hardening@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org, 
	"Gustavo A. R. Silva" <gustavoars@...nel.org>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 35/82] ACPI: custom_method: Refactor intentional
 wrap-around test

On Tue, Jan 23, 2024 at 2:03 AM Kees Cook <keescook@...omium.org> wrote:
>
> In an effort to separate intentional arithmetic wrap-around from
> unexpected wrap-around, we need to refactor places that depend on this
> kind of math. One of the most common code patterns of this is:
>
>         VAR + value < VAR
>
> Notably, this is considered "undefined behavior" for signed and pointer
> types, which the kernel works around by using the -fno-strict-overflow
> option in the build[1] (which used to just be -fwrapv). Regardless, we
> want to get the kernel source to the position where we can meaningfully
> instrument arithmetic wrap-around conditions and catch them when they
> are unexpected, regardless of whether they are signed[2], unsigned[3],
> or pointer[4] types.
>
> Refactor open-coded wrap-around addition test to use add_would_overflow().
> This paves the way to enabling the wrap-around sanitizers in the future.
>
> Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
> Link: https://github.com/KSPP/linux/issues/26 [2]
> Link: https://github.com/KSPP/linux/issues/27 [3]
> Link: https://github.com/KSPP/linux/issues/344 [4]
> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Len Brown <lenb@...nel.org>
> Cc: linux-acpi@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  drivers/acpi/custom_method.c | 2 +-

I may attempt to drop custom_method.c in this cycle, is there a
problem if I take this into my tree for now?

>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
> index d39a9b474727..0789317f4a1a 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -54,7 +54,7 @@ static ssize_t cm_write(struct file *file, const char __user *user_buf,
>
>         if ((*ppos > max_size) ||
>             (*ppos + count > max_size) ||
> -           (*ppos + count < count) ||
> +           (add_would_overflow(count, *ppos)) ||
>             (count > uncopied_bytes)) {
>                 kfree(buf);
>                 buf = NULL;
> --
> 2.34.1
>